1. Field of the Invention
The present invention relates to apparatus for establishing a relatively uniform charge level on charge-retaining materials, in general, and to such apparatus for establishing such a charge level on a moving web of such material, in particular.
2. Description of the Prior Art
The presence of electrostatic charges on charge-retaining materials causes problems in many industries. In the photographic industry, for example, electrostatic charges on potential photographs or film units within a light-tight film cassette containing a plurality of film units for use in an "instant" type photographic camera, such as that sold by Polaroid Corporation, Cambridge, Mass., under its registered trademark SX-70, will often cling to one another with such intensity as a result of the force of attraction developed by such electrostatic charges, that proper ejection of an exposed film unit from said film cassette can be prevented if the effects of such charges are not controlled. In the SX-70 photographic film mentioned above, for example, electrostatic charges are controlled by controlling the charge levels on components of said film prior to final film assembly.
Troublesome electrostatic charges on charge-retaining materials can be conveniently thought of as falling into two categories. One category is that of polarization charges, sometimes referred to as dipoles, and the other is that of free surface charges. Polarization charges are bound to a definite site in a solid, whereas free surface charges are not. Free surface charges on a moving web of certain materials, for example, are frequently neutralized by a grounded brush-like device such as that described in U.S. Pat. No. 3,757,164 to BINKOWSKI. Polarization charges in such a web are commonly controlled by subjecting the web to a corona-generated electrostatic field having the proper magnitude and polarity. It is often necessary to deal with both categories of charges.
Regulating polarization charges with a corona-generated electrostatic field is effective, but this technique has several disadvantages. A corona generates ozone gas and ozone can cause some individuals to become ill. Even if illness does not result from the ozone, it has a pungent odor that is objectional to most people, especially in an environment where ventilation is relatively poor or nonexistent. When a corona is employed to control polarization charges on a web of light-sensitive material, means must be provided to prevent light produced by the corona from reaching and thereby damaging said material. In addition, it is often necessary to expend significant quantities of relatively costly electric power in order to generate and maintain a corona-type electrostatic field for charge polarizing purposes.
Regulating polarization charges in charge retaining materials with a brush-like device having a small number of relatively coarse conductive bristles, as in U.S. Pat. No. 3,146,385 to CARLSON, may produce the desired charge level in portions of said material but the degree of charge level uniformity throughout said material would be relatively poor.